1. Field of the Invention
The present invention relates to the detection of exfoliation corrosion in metal structures, and in particular, to a method for detecting exfoliation corrosion in the aluminum wing and fuselage sections of aircraft structures.
2. Description of the Related Art
Over the years, many different nondestructive evaluation methods or inspection techniques have been used such as electromagnetic, thermal, ultrasonic, radiographic and optical methods to inspect for hidden cracks or corrosion in the wing and fuselage sections of aircraft.
Exfoliation corrosion refers to the physical appearance of a specific type of intergranular corrosion which is layered or leafed in character and consists of alternating strata of corroding and non-corroding metal. It typically occurs in high-strength aluminum-alloy rolled sheet or plate which has a laminar-like microstructure consisting of grains flattened in the plane of the sheet or plate. The exfoliation or corrosion attack occurs along the grain boundaries in the rolling plane and predominantly in the rolling direction of the aluminum sheet or plate.
Exfoliation corrosion is a very common form of corrosion in precipitation-hardened 7XXX series aluminum alloy (ALxe2x80x94Smxe2x80x94Mg) wrought products. Precipitation hardening, the process used to heat-treat aluminum alloys to the -T6 condition, tends to produce a somewhat continuous precipitate of Alxe2x80x94Zn, Alxe2x80x94Mg, and Mgxe2x80x94Zn intermetallic compounds in the grain boundaries of this series of alloys. These intermetallic compounds have an electrochemical potential anodic to the surrounding aluminum, hence causing them to preferentially corrode in certain corrosive environments. With the relative area of cathode (surrounding grains) to anode (small intermetallic particles) being quiet large, this attack can progress very rapidly with the grain boundary corrosion product wedging or separating the grains apart and causing the uncorroded material to lift or leaf. This effect produces the characteristic xe2x80x9cblisterxe2x80x9d appearance of exfoliation. Current generation aluminum alloys, 7050 and 7150, exhibit high strengths in -T6-type tempers and when they are processed to the -T76, -T74 or -T73-type temper, to improve their resistance to stress corrosion cracking and/or exfoliation corrosion although this improvement is often achieved at some cost to strength vis-a-vis the -T6 condition. However, using these tempers does not resolve the exfoliation corrosion problem for aluminum alloys, such as, 7178-T6 still in use today that were produced prior to this current generation of alloys.
The typical aircraft application for these alloys is on the upper wing skins where failure originating from exfoliation corrosion is a significant concern to prevent failure of the wing structure. Exfoliation corrosion detection is required for early and accurate detection on all aircraft. The exfoliation corrosion is sometimes evident on the surface of the wing as it can cause the paint coating to have a blistered appearance in the corroded areas. When detected, these areas are sanded to remove the gross exfoliation corrosion until it is no longer visually observable. However, exfoliation corrosion can still be present in the sanded areas that is not visually observable even at higher magnifications. It may also be present in areas where exfoliation corrosion was not previously observed and is not detectable on the surface. This xe2x80x9chidden exfoliationxe2x80x9d occurs when the amount of corrosion products between the grains is relatively small and has not caused the grains above the corroded boundaries to lift or separate from the subsurface grains.
Over the years, many different NDE methods or inspection techniques (electromagnetic, thermal, ultrasonic, radiographic and optical) have been developed and used to inspect for hidden cracks or corrosion in the wing and fuselage sections of aging aircraft. Each of these candidate techniques has its own inherent performance and detection limitations; therefore, new techniques or tools are being developed and validated for both production and field use to improve detection capability and reliability, and to reduce the cost of inspection.
Inspection for hidden exfoliation corrosion is typically conducted, for example, on the aircraft upper wing surface around fastener holes. The fastener holes are sometimes filled with steel rivets and a galvanic cell is created between the surface of the steel rivet and the surface of the hole in the aluminum wing skin, accelerating the rate of corrosion along the grain boundaries from the surface of the hole. There are several methods to locate the hidden exfoliation corrosion, but the current method utilizes a glass bead shot peening method and is called a search peening process. This search peening process is up to 95% efficient in detecting exfoliation corrosion. Once an area of corrosion has been detected, the area is sanded until the exfoliation corrosion is no longer visible in the sanded area. The area is then again glass bead search peened and inspected for additional exfoliation corrosion, revealed by the bead peening. This process is repeated until no more hidden exfoliation is detected after glass bead search peening.
Glass bead peening for inspection for exfoliation corrosion works by producing a compressive residual stress and cold work into the metal surface being inspected. These effects cause the surface material in areas having underlying grain boundary corrosion to exfoliate, i.e., blister or xe2x80x9cleaf up,xe2x80x9d and thereby expose only underlying corrosion. If there is no underlying corrosion, exfoliation does not occur.
Although generally effective, this method for exfoliation corrosion detection has several drawbacks. For example, it takes a significant amount of time to isolate the wing being search peened to contain the peening media and also to extricate the glass bead media from the surface being inspected. These requirements add three to six days to the Program Depot Maintenance (PDM) cycle for an aircraft. In addition to the added time, the additional workload increases the cost for maintenance of the aircraft.
Accordingly, the need exists for a cost-effective and time-efficient exfoliation corrosion detection apparatus and method which enables rapid inspection and evaluation of significant portions of the aircraft for exfoliation corrosion.
The present invention satisfies that need by providing an exfoliation corrosion detection method that enables rapid inspection for the detection and evaluation of exfoliation corrosion on aircraft with related cost savings.
The present invention utilizes laser shock peening to produce the compressive residual stresses and cold work into the metal surface, necessary to expose hidden exfoliation corrosion. Laser shock peening is currently being used in production to produce deep compressive residual stresses in titanium compressor blades and now this same technology can be adapted to search peening. The depth and magnitude of residual compressive stresses can be tailored by the application of specific overlays and controlling the intensity of the laser beam.
In the laser shock peening process, the surface of the part is first covered with two types of overlays, one transparent to the laser beam and the other opaque to the laser beam. The opaque overlay is applied directly to the surface of the part. The opaque overlay is typically paint or tape and has three functions. The first function is to protect the surface of the part from the intense heat of the plasma plume generated during the laser shock peening process. The second function is to enhance the strength of the shock wave from the plasma. The third and final function is to provide a consistent processing medium for the laser beam to couple to. The transparent overlay is typically water and is applied over the opaque overlay. The primary function of the transparent layer is to confine the plasma plume against the surface of the part in order to generate higher peak pressures during the laser shock peening process.
When the laser is fired, the laser beam passes through the transparent overlay and strikes the opaque overlay. When the laser beam strikes the opaque overlay, it initially vaporizes some of the opaque material surface. The rest of the laser beam is absorbed by the vaporized material and creates a plasma. The expanding plasma is confined between the transparent overlay and the surface of the part. As a result, high pressures up to 1,400,000 psi are generated in the confined plasma. The pressure exerted on the surface by the confined plasma generates a pressure pulse or shock wave that propagates into the metallic part. When the stress in the shock wave is above the dynamic elastic limit of the material, the surface material yields plastically. This plastic strain creates residual compressive stresses in the surface.
By varying the laser beam parameters and type of overlays, the magnitude and depth of the residual stresses may be controlled. Consequently, laser shock peening can be used to expose and identify hidden exfoliation corrosion, replacing the current search peening method that bombards the surface with glass beads.
Important parameters to the laser search peening process are the search pattern and the shape of the laser beam spot when searching around holes or along edges for exfoliation corrosion. The laser spot is the area laser peened by one pulse of the laser beam. One type of search pattern is a circular search pattern that can be used for laser search peening around holes, such as fastener holes. Another type of circular search pattern is a spiral or spiral. The third type of circular search pattern is a pattern of concentric rings. For these types of search patterns, individual laser spots are incremented a specific distance from either the edge of the hole or from the previous spot or from both the edge of the hole and from the previous spot. The laser beam spots may or may not overlap a neighboring spot. A single doughnut-shaped laser spot may be used to encircle a hole to provide up to 100 percent peening coverage of the desired search area, provided the power density is sufficient to introduce the needed residual stresses. Alternatively, the use of multiple laser spots may be needed to achieve 100 percent coverage of the desired area depending on the laser spot shape. The shape of the laser beam spot on the part may have many other configurations. The shape may be round, square or elliptical, or other variations, in addition to the doughnut or equivalent shape. Each of these shapes may be used to provide the most efficient laser search peening.
In addition to searching around holes, areas near other types of machined or cut edges can also contain exfoliation corrosion. Three search patterns can be used for searching these areas. The first type of search pattern is a laser spot straddling a joint or edge to simultaneously search peen the surface on both sides of the joint or edge. The second type of search pattern is to search peen with individual laser spots on each side of the joint. Finally, laser spots on one side only can be used if only one side is to be inspected. The shape of the laser spots can be of any shape to provide efficient laser search peening.
The laser search peen process may be automated by mounting the laser search peen equipment on a remotely controlled robotic vehicle, such as an AutoCrawler M50, wherein the AutoCrawler M50, manufactured by Advance Robotic Vehicles, Inc. of Seattle, Wash., is capable of walking or crawling along the surface of the part being search peened and positioning itself in a specific location. The equipment to be mounted in the robotic vehicle may include the laser delivery head, laser beam effluent removal system, imaging system, and other support systems.
Accordingly, it is an object of the present invention to provide a search peening method which may be applied to exfoliation corroded surfaces of various types, curved or flat.
It is another object of the present invention to provide a search peening method which overcomes the long preparation and cleanup required during the use of the current state of the art glass bead shot peening process.
It is a further object of the present invention to provide a search peening method for detecting hidden exfoliation corrosion with greater precision, accuracy and speed than existing methods.
It is a further object of the present invention to provide a search peening method for detecting hidden exfoliation corrosion, which is easy to use and adaptable for field use.
It is a further object of the present invention to provide a search peening method for detecting hidden exfoliation corrosion where the entire wing structure to be processed will not need to be isolated from the rest of the repair or service facility during the search process.
It is a still further object of the present invention to reduce the time and cost for detecting hidden exfoliation corrosion. These and other objects, features and advantages of the present invention will be apparent from the drawings, detailed description and claims which follow.